A general view of atmosphere during Electric Zoo 2013 at Randall's Island in New York City. (Aug. 31, 2013) Photo Credit: Getty Images

Two deaths, apparently from drug overdoses, at the Electric Zoo dance music festival on Randalls Island Park caused concert organizers and city officials to cancel Sunday's events.

Two concertgoers attending what was to have been a three-day fest off upper Manhattan apparently overdosed Saturday and later died, authorities said. At least four others became critically ill and are in intensive care units at hospitals.

Police said that definitive causes of the deaths have not been determined, but appear to have been caused by the drug MDMA, also known as ecstasy or "molly."

Jeffrey Russ, 23, of Rochester, was taken to Harlem Hospital Center, where he was pronounced dead. Olivia Rotundo, 20, of Rhode Island, was taken to Metropolitan Hospital Center, where she was pronounced dead.

The ill concertgoers at the outdoor electronic dance festival were taken to hospitals by private ambulance crews on duty at the event, according to people close to festival organizers. The FDNY said it did not respond to emergency calls at the concert.

Last October, Nassau police shut down a similar electronic music rave party at Nassau Coliseum after 50 people at the "Haunted Coliseum" event became ill. Police said they responded to as many as 100 calls of intoxicated young people at the Uniondale event, where witnesses reported seeing people using ecstasy and drinking alcohol in soda bottles. The Nassau festival and the Randalls Island event have no connection.

The Electric Zoo festival has been held the past four years over Labor Day weekend at Randalls Island Park. On Sunday, in announcing the cancellation, organizers of the festival, which features a variety of DJs and live acts with a focus on rave music and dancing, said they "send our deepest condolences to the families of the two people who passed away this weekend. Because there is nothing more important to us than our patrons, we have decided in consultation with the New York City Parks Department that there will be no show today," according to the statement on their website.

On their Facebook page, festival officials wrote that information about refunds would be posted later.

In the city's statement Sunday, officials in Mayor Michael Bloomberg's office said "the Electric Zoo organizers have worked with City officials to reduce health risks at this event, but in view of these occurrences, the safest course is to cancel the remaining day of the event."